Microwave signals are transmitted from point-to-point by waveguides or antennas. The main difference between the two is that an antenna radiates the electromagnetic field into open space, while a waveguide confines the electromagnetic field to an area along the signal path. There are a number of conventional waveguide sub-categories.
A transmission line is one such sub-category. It uses some physical configuration of metal and/or dielectrics to direct a signal along the desired path. Typical transmission lines use two conductors—a signal and ground. There are also single conductor transmission lines, such as rectangular waveguides. The simplest type of transmission line configuration is coaxial line. Stripline is essentially a flattened version of the coaxial line configuration. Microstrip line simplifies the stripline configuration, by removing the upper ground planes.
Microstrip line is generally the most commonly used means for planar transmission line applications, because it is highly manufacturable and eases circuit connections and signal probing. Its disadvantage over stripline is that some of the energy transmitted may couple to adjacent traces or into space. Unlike microwave antennas, microwave waveguides are not intended to radiate energy in this way, which causes signal loss and interference. A coplanar waveguide with ground (CPWG) is essentially a low radiation version of microstrip.
In any case, as a microwave signal travels down the signal path of a waveguide, it is subjected to various characteristics associated with that path, such as path discontinuity. Such characteristics generally impact on the waveguides performance, and must be taken into account. For example, consider the case where a microwave signal must be routed through a printed wiring board. Passing from one layer of the printed-circuit board to another can be achieved with metallic pads on each layer, and a hole that is plated with metal to connect these pads. Such structures are commonly called vias. The via pad is typically surrounded by a ground plane, with a ground gap in between.
The problem here is that standard processing of printed wiring boards does not accommodate the particularities associated with microwave applications in high density RF environments. For instance, IPC, formerly known as the Institute for Printed Circuits, is an organization that provides standards in the electronic interconnection industry. Generally, via structures must be made according to design rules that accommodate IPC standard processing techniques. Often times, however, the minimum pad size for a given via diameter is significantly larger than the transmission line conductor that attaches to it. Such discontinuity ultimately limits device performance at high microwave frequencies (e.g., greater than 2 GHz).
What is needed, therefore, are techniques for transitioning high frequency signals on a printed wiring board processed in accordance with industry standards, such as the IPC specifications.